Strains of dogs with hereditary lesion-specific defects in cardiovascular development are used to study the pathogenesis of conotruncal septum defects (including tetralogy of Fallot, ventricular septal defect, and persistent truncus arteriosus), pulmonary valve dysplasia, and patent ductus arteriosus. Comparing embryos from affected strains with normal control embryos, studies progress through classical descriptive embryologic studies of abnormalities in shape and form of the developing heart, to investigation of the roles of cell proliferation and differential cell death in the genesis of these malformations. Methods used include wax plate reconstruction of the developing heart from serial sections and scanning electron microscopy of microdissections at critical stages in development, quantitative morphometric studies of mitotic activity and cell death using cytologic techniques, as well as autoradiographic assessment of incorporation of tritiated thymidine into DNA in areas of abnormal development.